The field of the present invention pertains to semiconductor fabrication processing. More particularly, the present invention relates to a device for more efficiently utilizing slurry for polishing a semiconductor wafer in a chemical mechanical polishing machine.
Most of the power and usefulness of today""s digital IC devices can be attributed to the increasing levels of integration. More and more components (resistors, diodes, transistors, and the like) are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal. It takes the shape of a solid cylinder. This crystal is then sawed (like a loaf of bread) to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components are commonly defined photographically through a process known as photolithography. Very fine surface geometries can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built-up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex IC""s often resemble familiar terrestrial xe2x80x9cmountain ranges,xe2x80x9d with many xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern, defining the various components, is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using the optical means of the photolithography tool, and is imprinted into the photosensitive layer. To build ever smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer, e.g. optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, the xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleys,xe2x80x9d exaggerate the effects of decreasing depth of focus. Thus, in order properly to focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (e.g. fully planarized) surface will allow for extremely small depths of focus, and in turn, allow the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) is the preferred method of obtaining full planarization of a wafer. It involves removing a sacrificial layer of dielectric material using mechanical contact between the wafer and a moving polishing pad with chemical assistance from a polishing slurry. Polishing flattens out height differences, since high areas of topography (hills) are removed faster than areas of low topography (valleys). Polishing is the only technique with the capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
FIG. 1A shows a down view of a CMP machine 100 and FIG. 1B shows a side cut away view of the CMP machine 100 taken through line AA. The CMP machine 100 is fed wafers to be polished. The CMP machine 100 picks up the wafers with an arm 101 and places them onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is textured, often with a plurality of predetermined groves 103, to aid the polishing process. The polishing pad 102 rotates on a platen 104, or turn table located beneath the polishing pad 102, at a predetermined speed. A wafer 105 is held in place on the polishing pad 102 and the arm 101 by a carrier ring 112 and a carrier 106. The lower surface of the wafer 105 rests against the polishing pad 102. The upper surface of the wafer 105 is against the lower surface of the carrier 106 of the arm 101. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 into the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense arm 107 extending across the radius of the polishing pad 102. The slurry dispense arm 107 dispenses a flow of slurry onto the polishing pad 102.
CMP machine 100 also includes a conditioner assembly 120, which includes a conditioner arm 108 extending across the radius of the polishing pad 102. An end effector 109 is connected to the conditioner arm 108. The end effector 109 includes an abrasive conditioning disk 110 which is used to roughen the surface of the polishing pad 102, thereby improving the transport of slurry to and from wafer 105.
The slurry is a mixture of de ionized water and polishing agents designed to aid chemically the smooth and predictable planarization of the wafer. The rotating actions of both the polishing pad 102 and the wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafer 105 at some nominal rate. This rate is referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from surface topography. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, degrading wafer through-put of the fabrication process. If the removal rate is too fast, the CMP planarization process will not be uniform across the surface of the wafers, degrading the yield of the fabrication process.
Referring still to FIG. 1A and FIG. 1B, the polishing action of the slurry largely determines the removal rate and removal rate uniformity, and, thus, the effectiveness of the CMP process. As slurry is xe2x80x9cconsumedxe2x80x9d in the polishing process, the transport of fresh slurry to the surface of the wafer 105 and the removal of polishing by-products away from the surface of the wafer 105 become very important in maintaining the removal rate. Slurry transport is facilitated by the texture of the surface of the polishing pad 102. This texture is comprised of both predefined pits and grooves 103 that are manufactured into the surface of the polishing pad 102 and the inherently rough surface of the material from which the polishing pad 102 is made.
Referring now to FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D, the relationships between a wafer, a carrier ring, and a polishing pad are shown (for teaching purposes, the above elements are not necessarily drawn to scale). FIG. 2A and FIG. 2B show a wafer 105 and a carrier ring 112 respectively. FIG. 2C and FIG. 2D show a side view of the wafer 105 in the carrier ring 112 on a polishing pad 102. As described above, the wafer 105 is held in place on the arm (not shown) by the carrier ring 112 as the polishing pad 102 rotates on the polishing platen. The carrier ring 112 accepts the wafer 105 within its inner radius surface 201. The upper surface of the wafer 105 is against the carrier 106 (not shown) of the arm. The carrier 106 (not shown) presses the wafer into the polishing pad with a predetermined force. As the polishing pad 102 rotates, carrier 106 (not shown) rotates the wafer 105.
Referring still to FIG. 2D, the wafer 105 typically protrudes slightly, relative to the lower surface of carrier ring 112. This gives the polishing pad 102 and the slurry (not shown) on the polishing pad 102 an even contact with wafer 105. The carrier ring 112 holds the wafer 105 in place while the polishing pad 102 and the slurry polish the wafer 105. Polishing pad 102 frictionally slides against the lower surface of carrier ring 112 and against wafer 105. The predetermined amount of down force increases the friction between polishing pad 102, carrier ring 112, and wafer 105, thus increasing the removal rate. As depicted in FIG. 2D, wafer 105 protrudes by a positive protrusion amount past the lower surface of the carrier ring 112. This gives the polishing pad 102 and the slurry (not shown) on the polishing pad 102 less obstructed contact with the wafer 105. The carrier ring 112 inherently obstructs a certain amount of slurry flow onto and under the wafer 105. The carrier ring 112 must hold the wafer 105 in place while the polishing pad 102 and the slurry polish the wafer 105. Even though the carrier ring 112 obstructs a certain amount of slurry flow, enough slurry contacts the wafer 105 to complete a polishing cycle.
The problem, however, is that the period of time required to complete the polishing cycle is increased due to the inherent obstruction of slurry flow to the wafer by the carrier ring. In a typical CMP machine (e.g., CMP machine 100), the slurry is dispensed from the slurry dispense arm 107 onto polishing pad 102, as polishing pad 102 rotates. The slurry spreads nearly uniformly across the surface of polishing pad 102 due to the movement and action of the CMP machine 100 (e.g., centrifugal force, movement of wafer 105 and carrier ring 112 by arm 101, etc.). Only a small portion if the slurry dispensed by slurry dispense arm 107 ever comes into contact with wafer 105. The majority of the slurry is wasted, as it eventually flows off of polishing pad 102.
Slurry represents the most expensive consumable used in the CMP process. As described above, the CMP process uses an abrasive slurry on a polishing pad. The polishing action of the slurry is comprised of an abrasive frictional component and a chemical component. The abrasive frictional component is due to the friction between the surface of the polishing pad, the surface of the wafer, and abrasive particles suspended in the slurry. The chemical component is due to the presence in the slurry of polishing agents which chemically interact with the material of the dielectric layer. The chemical component of the slurry is used to soften the surface of the dielectric layer to be polished, while the frictional component removes material from the surface of the wafer.
The constituents of the slurry are precisely determined and controlled in order to effect the most optimal CMP planarization. Differing slurries are used for differing layers of the semiconductor wafer, with each slurry having specific removal characteristics for each type of layer. As such, slurries used in extremely precise sub-micron processes (e.g., tungsten damascene planarization) can be very expensive. Accordingly, the wasting of such slurry is to be avoided where ever possible.
One prior art solution to this problem involves slurry reuse, where the slurry which flows off of the polishing pad (e.g., polishing pad 102) is removed (e.g., by suction, drainage, etc.) and recycled via filtration or other similar means. The problem with this solution is that the removed slurry is typically contaminated with polishing by-product. Filtration and other such means may not be sufficient to recycle fully the potency of the slurry. For example, some contaminants may remain after the filtration, or one or more of the chemical components of the slurry may be consumed.
Thus what is required is a device which reduces the waste of slurry in the CMP process of a CMP machine. What is required is a device which reduces the amount of wasted slurry without the drawbacks of prior art slurry recycling schemes. What is further required is a device which renders the CMP process more cost effective by using slurry in the most efficient manner. The present invention provides a novel solution to the above requirements.
The present invention provides a device that reduces the waste of slurry in the CMP process of a CMP machine. The present invention provides a device that reduces the amount of wasted slurry without the drawbacks of prior art slurry recycling schemes. In addition, the present invention provides a device that renders the CMP process more cost effective by using slurry in the most efficient manner.
In one embodiment, the present invention comprises a slurry dispensing carrier ring for confining a semiconductor wafer to a polishing pad in a chemical mechanical polishing machine. The slurry dispensing ring has a diameter, a lower surface substantially parallel to the plane defined by the diameter, and an inner radius surface substantially orthogonal to the plane defined by the diameter. The inner radius surface is adapted to confine the semiconductor wafer. An outer radius surface is located opposite the inner radius surface. An upper surface is located opposite the lower surface.
A plurality of slurry dispense holes extends through the carrier ring from the upper surface to the lower surface, wherein the slurry dispense holes are adapted to flow a slurry used for chemical mechanical polishing from the CMP machine to the lower surface so that the slurry contacts the semiconductor wafer confined within the inner radius surface. This provides for the more efficient utilization of slurry in the CMP process wherein a planar topography is created on the semiconductor wafer. This facilitates the subsequent semiconductor processing steps performed on said semiconductor wafer and minimizes the amount of wasted slurry, thereby rendering the CMP process more cost effective by using slurry in the most efficient manner.
The precisely metered and targeted delivery of slurry minimizes the exposure of the slurry to the atmosphere, thereby minimizing any possible contamination or degradation of the slurry due to contact with atmospheric gasses (e.g., oxygen). The targeted delivery of slurry also enhances the ability of the CMP machine to regulate precisely the temperature of the slurry as it is used in the CMP process.